1. Field of the Invention
The present invention relates to an ultrasonic flow meter that measures volume flow or mass flow of fluid using an ultrasonic wave.
2. Description of the Related Art
JP-A-56-67716 discloses an ultrasonic flow meter that uses a propagation time difference method FIG. 7 is a diagram showing the ultrasonic flow meter. The ultrasonic flow meter includes devices 91 and 92 disposed on a sidewall of a pipe 90 in which fluid flows. The device 91 is disposed upstream in a flow direction of the fluid and the device 92 is disposed downstream in the flow direction of the fluid. The devices 91 and 92 are obliquely arranged so that a radiation axis of an ultrasonic wave crosses the flow direction of the fluid at an acute angle.
When a propagation time of an ultrasonic wave from the device 91 to the device 92 is indicated by “tA,” a propagation time of the ultrasonic wave from the device 92 to the device 91 is indicated by “tB,” a distance between the devices 91 and 92 is indicated by “L,” a sound speed is indicated by “C,” a flow rate of the fluid is indicated by “V,” and an angle between the flow direction and the radiation axis of the ultrasonic wave is indicated by “θ” and when the flow rate V=0, the relationship of tA=tB=L/C is satisfied. When V>0, a relationship of tA=L/(C+V cos θ) and tB=L/(C−V cos θ) is satisfied. That is, V=L((1/tA)−(1/tB))/2 cos θ. When a cross-sectional area of the pipe 90 is indicated by “S” the volume flow “Q” is calculated from Q=SV.
The sound speed depends on a pressure of the fluid and a humidity of the fluid. Thus, in order to calculate the volume flow with accuracy, it is required to correct the volume flow based on the pressure of the fluid and the humidity of the fluid. WO 2007/020113 A1 (corresponding to US 2009/0178490 A1) discloses an ultrasonic flow meter that measures a pressure of fluid and a humidity of the fluid by providing an ultrasonic sensor, a pressure sensor, and a temperature sensor on a flow channel and corrects a volume flow based on the measured values. The temperature sensor is used when the sound speed is corrected with the humidity.
Because the above-described ultrasonic flow meter needs the pressure sensor and the temperature sensor, a dimension of the ultrasonic flow meter increases, and an installing space of the ultrasonic flow meter increases. In addition, a manufacturing cost of the ultrasonic flow meter increases.